One option would be to have a dealer configure the fill timer to twice what you think you need. That way you just set it to 50% and if you change bags from 750# to 1100#, you just adjust the 2012 touch screen to 67.5%.

Small edit:

In other words your rear port ballast is 250# let's say that is 2500 revolutions, the tech term is 'pulse counts', of the pump... That is just a guess and just for this example...

When you go to the dealer because you want to add a 750# bag, have them configure it for 2500+7500 = 10,000 x 2 = 20,000 to read 100% full on the medallion screen....I believe the same applies to those 2012 without the touch screen as well. So now at 50% full your pump would have run 10k revolutions and your 750# plus the stock 250# will be full. Then when you change to an 1100# bag, you'll need 11,000+2500 = 13,500 revolutions (pulse counts) to fill... Or 67.5% of the 20,000 your dealer configured.

In this configuration, I would advise you to vent the bags with a check valve, that way you can determine what % was needed to over fill the total ballast.